1. Field of the Invention
The present invention relates to a thermosensitive recording material, and more particularly to a thermosensitive recording material capable of producing images with excellent preservation stability, comprising as the main components a leuco dye serving as a coloring agent and a color developer capable of inducing color formation in the leuco dye upon application of heat thereto.
2. Discussion of Background
There are conventionally proposed various recording materials which utilize the coloring reaction between a colorless or light-colored electron donor type dye such as a leuco dye and a color developer capable of inducing color formation in the leuco dye upon application of heat or pressure thereto when brought into contact with the leuco dye.
A thermosensitive recording material, one of the above-mentioned recording materials, is proposed as disclosed in Japanese Patent Publications 43-4160 and 45-14039 and Japanese Laid-Open Patent Application 48-27736. Such a thermosensitive recording material is usable as a recording material for an electronic computer, facsimile apparatus, ticket vending apparatus, label printer, and recorder because it has the advantages that complicated processes such as development and image-fixing are not required, recording can be achieved for a short period of time using a relatively simple apparatus, there is no noise development, and the manufacturing cost is low.
In such a thermosensitive recording material, colorless or light-colored leuco dyes having a lactone, lactam, or a spiropyran ring are used as coloring dyes, and organic acids or phenols are conventionally employed as color developers. The thermosensitive recording material using the above-mentioned leuco dye and color developer is widely used for practical use because the produced images have high image density, with the whiteness of the background maintained high.
In line with the increase of a demand for the thermosensitive recording system, the requirements for improved preservability of the images recorded on the thermosensitive recording material with respect to the resistances to chemicals, light and heat are increased. Therefore, the development of a recording material capable of meeting the above-mentioned requirements is intensively desired.
To improve the preservation stability of the recorded images on the thermosensitive recording material, it is proposed to use as the color developer with high reliability a phenolsulfonic acid compound, as disclosed in Japanese Laid-Open Patent Applications 58-82788 and 60-13852; a metallic salt of benzoic acid, as disclosed in Japanese Laid-Open Patent Application 61-47292; and a substituted salicylic acid compound, as disclosed in Japanese Laid-Open Patent Application 62-169681. However, the fastness to fats and oils, plasticizers, and water of the images recorded on the thermosensitive recording materials comprising the above-mentioned color developers is still insufficient.
The formation of an intermediate layer is proposed to increase the recording sensitivity of the thermosensitive recording material. For instance, there are proposed an intermediate layer mainly comprising a variety of inorganic pigments, and an intermediate layer comprising non-expandable void particles. However, these recording materials have the shortcomings that the thermal coloring sensitivity is insufficient and the recording properties are poor.
Accordingly, a first object of the present invention is to provide a thermosensitive recording material with high thermal coloring sensitivity and good recording properties, capable of coping with high-speed thermal recording, and producing images with improved resistances to fats and oils, and plasticizers to ensure the preservation stability.
A second object of the present invention is to provide a color developer for use in the thermosensitive recording material.
The above-mentioned first object of the present invention can be achieved by a thermosensitive recording material comprising a support and a thermosensitive recording layer formed on the support, comprising a leuco dye and a color developer capable of inducing color formation in the leuco dye upon application of heat thereto, the color developer comprising at least one compound selected from the group consisting of compounds represented by formulas (I) to (IV): 
wherein R1 is a saturated or unsaturated chain hydrocarbon group having 1 to 10 carbon atoms which may be branched, a chain hydrocarbon group having 1 to 10 carbon atoms which may be substituted by hydroxyl group or interrupted by an ether linkage or carbonyl group, an aryl group selected from the group consisting of phenyl group and naphthyl group, which may have a substituent selected from the group consisting of an alkyl group having 1 to 6 carbon atoms, an alkoxyl group having 1 to 6 carbon atoms, nitro group or a halogen, 
in which X is a saturated or unsaturated bivalent hydrocarbon group having 1 to 10 carbon atoms, or a bivalent hydrocarbon group having 1 to 10 carbon atoms which may be substituted by hydroxyl group or interrupted by an ether linkage or carbonyl group; and Y is an alkyl group having 1 to 6 carbon atoms, an alkoxyl group having 1 to 6 carbon atoms, nitro group or a halogen; and l is an integer of 0 to 5; 
wherein R2 is a saturated or unsaturated chain hydrocarbon group having 1 to 10 carbon atoms which may be branched, a chain hydrocarbon group having 1 to 10 carbon atoms which may be substituted by hydroxyl group or interrupted by an ether linkage or carbonyl group, an aryl group selected from the group consisting of phenyl group and naphthyl group, which may have a substituent selected from the group consisting of an alkyl group having 1 to 6 carbon atoms, an alkoxyl group having 1 to 6 carbon atoms, nitro group or a halogen, 
in which X is a saturated or unsaturated bivalent hydrocarbon group having 1 to 10 carbon atoms, or a bivalent hydrocarbon group having 1 to 10 carbon atoms which may be substituted by hydroxyl group or interrupted by an ether linkage or carbonyl group; and Y is an alkyl group having 1 to 6 carbon atoms, an alkoxyl group having 1 to 6 carbon atoms, nitro group or a halogen; and l is an integer of 0 to 5; 
wherein R3 is a saturated or unsaturated chain hydrocarbon group having 1 to 10 carbon atoms which may be branched, a chain hydrocarbon group having 1 to 10 carbon atoms which may be substituted by hydroxyl group or interrupted by an ether linkage or carbonyl group, an aryl group selected from the group consisting of phenyl group and naphthyl group, which may have a substituent selected from the group consisting of an alkyl group having 1 to 6 carbon atoms, an alkoxyl group having 1 to 6 carbon atoms, nitro group or a halogen, 
in which X is a saturated or unsaturated bivalent hydrocarbon group having 1 to 10 carbon atoms, or a bivalent hydrocarbon group having 1 to 10 carbon atoms which may be substituted by hydroxyl group or interrupted by an ether linkage or carbonyl group; and Y is an alkyl group having 1 to 6 carbon atoms, an alkoxyl group having 1 to 6 carbon atoms, nitro group or a halogen; and l is an integer of 0 to 5; and 
wherein R4 is a saturated or unsaturated chain hydrocarbon group having 1 to 10 carbon atoms which may be branched, a chain hydrocarbon group having 1 to 10 carbon atoms which may be substituted by hydroxyl group or interrupted by an ether linkage or carbonyl group, an aryl group selected from the group consisting of phenyl group and naphthyl group, which may have a substituent selected from the group consisting of an alkyl group having 1 to 6 carbon atoms, an alkoxyl group having 1 to 6 carbon atoms, nitro group or a halogen, 
in which X is a saturated or unsaturated bivalent hydrocarbon group having 1 to 10 carbon atoms, or a bivalent hydrocarbon group having 1 to 10 carbon atoms which may be substituted by hydroxyl group or interrupted by an ether linkage or carbonyl group; and Y is an alkyl group having 1 to 6 carbon atoms, an alkoxyl group having 1 to 6 carbon atoms, nitro group or a halogen; and l is an integer of 0 to 5.
Furthermore, the first object of the present invention can also be achieved by a thermosensitive recording material comprising a support, an intermediate layer comprising spherical minute void particles comprising a thermoplastic resin, and a thermosensitive recording layer formed on the intermediate layer, comprising a leuco dye and a color developer capable of inducing color formation in the leuco dye upon application of heat thereto, the color developer comprising at least one compound selected from the group consisting of the above-mentioned compounds represented by formulas (I) to (IV).
In addition, the second object of the present invention can be achieved by novel phthalic acid derivatives of formulas (V) and (VI); 
The thermosensitive recording layer of the thermosensitive recording material according to the present invention comprises a color developer which comprises at least one compound selected from the group consisting of the above-mentioned compounds of formulas (I) to (IV), so that the preservation stability of the recorded images is improved, in particular, with respect to the resistances to fats and oils, and plasticizers.
The thermosensitive recording material of the present invention may further comprise an intermediate layer comprising as the main component plastic void particles in the form of sphere, which is provided between the support and the thermosensitive recording layer. The thermosensitivity of the recording material is improved owing to such an intermediate layer, so that the thermosensitive recording material becomes suitable for high-speed thermal recording.
In the present invention, the color developer for use in the thermosensitive recording layer comprises at least one compound selected from the group consisting of compounds of formulas (I) to (IV).
The chain hydrocarbon group in the definition of R1 to R4 in formulas (I) to (IV) have 1 to 10 carbon atoms, preferably 3 to 10 carbon atoms.
Specific examples of the compound of formula (I) are shown in Table 1:
Specific examples of the compound of formula (II) are shown in Table 2:
Specific examples of the compound of formula (III) are shown in Table 3:
Specific examples of the compound of formula (IV) are shown in Table 4:
In the case where the intermediate layer comprising as the main component plastic minute void particles is interposed between the support and the thermosensitive recording layer, the intermediate layer serves as a heat insulating layer, so that the thermal energy supplied by heat-application means such as a thermal head can be utilized efficiently to improve the thermosensitivity of the recording material.
The void particles for use in the intermediate layer comprise a thermoplastic resin for forming a shell of each void particle. A copolymer resin mainly comprising vinylidene chloride and acrylonitrile is preferably used as the above-mentioned thermoplastic resin. Air or other gasses are contained in the void particles in the expanded state.
It is preferable that the average particle diameter of the void particles be in the range from 2 to 10 xcexcm in the present invention. When the particle size of the void particles is within the above range, there is no problem in the production of the intermediate layer because the voidage of the void particles can freely be determined. In addition, the surface smoothness of the intermediate layer is not decreased although it is prepared by coating a coating liquid comprising such void particles and drying the same, so that the adhesion of the recording layer to the thermal head does not lower, and consequently, the thermosensitivity of the recording material can be prevented from deteriorating. When the above-mentioned advantages are further taken into consideration, it is preferable that the void particles classified in a narrow distribution be employed for use in the intermediate layer.
It is preferable that the voidage of the void particles for use in the intermediate layer be 50% or more, and more preferably 90% or more, from the viewpoint of the heat insulating effect. In the present invention, the voidage of the void particles for use in the intermediate layer is expressed by the following formula:       Voidage    ⁢          xe2x80x83        ⁢          (      %      )        =                    (                  inner          ⁢                      xe2x80x83                    ⁢          diameter          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          void          ⁢                      xe2x80x83                    ⁢          particles                )                    (                  outer          ⁢                      xe2x80x83                    ⁢          diameter          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          void          ⁢                      xe2x80x83                    ⁢          particles                )              xc3x97    100  
When the voidage of the void particles is within the above range, sufficient heat insulating effect of the intermediate layer can be obtained, so that the thermal energy supplied by the thermal head is prevented from escaping through the support of the thermosensitive recording material. As a result, the thermosensitivity-improving effect can be increased. In the present invention, when the intermediate layer comprises void particles with an average particle diameter of 2 to 10 xcexcm and a voidage of 90% or more, the flexibility of the obtained recording material is so much increased that the adhesion to the thermal head is further increased, thereby improving the dot reproduction performance.
The intermediate layer for use in the present invention may further comprise an inorganic or/and organic pigment. In this case, the oil absorption of the pigment is preferably 30 ml/100 g or more, and more preferably 80 ml/100 g or more.
The above-mentioned inorganic and/or organic pigment used in the intermediate layer, which may be employed alone or in combination, can be selected from any pigments for use in the conventional thermosensitive recording materials. Specific examples of the inorganic pigment are calcium carbonate, silica, zinc oxide, titanium oxide, aluminum hydroxide, zinc hydroxide, barium sulfate, clay, talc, and surface-treated calcium and silica. Specific examples of the organic pigment are urea-formaldehyde resin, styrene-methacrylic acid copolymer and polystyrene resin.
The thermosensitive recording layer of the recording material according to the present invention comprises a leuco dye serving as a coloring agent.
As the leuco dye for use in the present invention, which may be employed alone or in combination, any conventional dyes for use in the conventional leuco-dye-containing recording materials can be employed. For example, triphenylmethanephthalide leuco compounds, triallylmethane leuco compounds, fluoran leuco compounds, phenothiazine leuco compounds, thiofluoran leuco compounds, xanthene leuco compounds, indophthalyl leuco compounds, spiropyran leuco compounds, azaphthalide leuco compounds, couromeno-pyrazole leuco compounds, methine leuco compounds, rhodamineanilinolactam leuco compounds, rhodaminelactam leuco compounds, quinazoline leuco compounds, diazaxanthene leuco compounds and bislactone leuco compounds are preferably employed. Specific examples of those leuco dyes are as follows:
3,3-bis(p-dimethylanilino)phthalide,
3,3-bis(p-dimethylanilino)-6-dimethylaminophthalide (or Crystal Violet Lactone),
3,3-bis(p-dimethylanilino)-6-diethylaminophthalide,
3,3-bis(p-dimethylanilino)-6-chlorophthalide,
3,3-bis(p-dibutylanilino)phthalide,
3-cyclohexylamino-6-chlorofluoran,
3-dimethylamino-5,7-dimethylfluoran,
3-diethylamino-7-chlorofluoran,
3-diethylamino-7-methylfluoran,
3-diethylamino-7,8-benzfluoran,
3-diethylamino-6-methyl-7-chlorofluoran,
3-(N-p-tolyl-N-ethylamino)-6-methyl-7-anilinofluoran,
3-pyrrolidino-6-methyl-7-anilinofluoran,
2-(m-trifluoromethylanilino)-6-diethylaminofluoran,
2-[3,6-bis(diethylamino)-9-(o-chloroanilino)xanthylbenzoic acid lactam],
3-diethylamino-6-methyl-7-(m-trichloromethylanilino)fluoran,
3-diethylamino-7-(o-chloroanilino)fluoran,
3-dibutylamino-7-(o-chloroanilino)fluoran,
3-diamylamino-6-methyl-7-anilinofluoran,
3-(N-methyl-N-amylamino)-6-methyl-7-anilinofluoran,
3-(N-methyl-N-isopropylamino)-6-methyl-7-anilinofluoran,
3-(N-methyl-N-cyclohexylamino)-6-methyl-7-anilinofluoran,
3-(N-ethyl-N-isopropylamino)-6-methyl-7-anilinofluoran,
3-(N-methyl-N-isoamylamino)-6-methyl-7-anilinofluoran,
3-(N-methyl-N-isobutylamino)-6-methyl-7-anilinofluoran,
3-diethylamino-6-chloro-7-anilinofluoran,
3-(N-ethyl-N-2-ethoxypropylamino)-6-methyl-7-anilinofluoran,
3-(N-ethyl-N-tetrafurfurylamino)-6-methyl-7-anilinofluoran,
3-(N-ethyl-N-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran,
3-diethylamino-6-methyl-7-anilinofluoran,
3-dibutylamino-6-methyl-7-anilinofluoran,
3-diethylamino-5-methyl-7-(N,N-dibenzylamino)fluoran,
benzoyl leuco methylene blue,
6xe2x80x2-chloro-8xe2x80x2-methoxy-benzoindolino-spiropyran,
6xe2x80x2-bromo-8xe2x80x2-methoxy-benzoindolino-spiropyran,
3-(2xe2x80x2-hydroxy-4xe2x80x2-dimethylanilino)-3-(2xe2x80x2-methoxy-5xe2x80x2-chlorophenyl)phthalide,
3-(2xe2x80x2-hydroxy-4xe2x80x2-dimethylanilino)-3-(2xe2x80x2-methoxy-5xe2x80x2-nitrophenyl)phthalide,
3-(2xe2x80x2-hydroxy-4xe2x80x2-diethylanilino)-3-(2xe2x80x2-methoxy-5xe2x80x2-tolyl)phthalide,
3-diethylamino-6-methyl-7-(2xe2x80x2,4xe2x80x2-dimethylanilino)fluoran,
3-(2xe2x80x2-methoxy-4xe2x80x2-dimethylanilino)-3-(2xe2x80x2-hydroxy-4xe2x80x2-chloro-5xe2x80x2-tolyl)phthalide,
3-morphorino-7-(N-propyl-trifluoromethylanilino)fluoran,
3-pyrrolidino-7-trifluoromethylanilinofluoran,
3-diethylamino-5-chloro-7-(N-benzyl-trifluoromethylanilino)fluoran
3-pyrrolidino-7-(di-p-chlorophenyl)methylaminofluoran,
3-diethylamino-5-chloro-7-(xcex1-phenylethylamino)fluoran,
3-(N-ethyl-p-toluidino)-7-(xcex1-phenylethylamino)fluoran,
3-diethylamino-7-(o-methoxycarbonylphenylamino)fluoran,
3-diethylamino-5-methyl-7-(xcex1-phenylethylamino)fluoran,
3-diethylamino-7-piperidinofluoran,
2-chloro-3-(N-methyltoluidino)-7-(p-N-butylanilino)fluoran,
3-(N-ethyl-N-cyclohexylamino)-5,6-benzo-7-xcex1-naphtylamino-4xe2x80x2-bromofluoran,
3-(N-benzyl-N-cyclohexylamino)-5,6-benzo-7-xcex1-naphthylamino-4xe2x80x2-bromofluoran,
3-diethylamino-6-methyl-7-mesidino-4xe2x80x2,5xe2x80x2-benzofluoran,
3-(p-dimethylanilino)-3-[1,1-bis(p-dimethylanilino)ethylene-2-yl]phthalide,
3-(p-dimethylanilino)-3-[1,1-bis(p-dimethylanilino)ethylene-2-yl]-6-dimethylaminophthalide,
3-(p-dimethylanilino)-3-(1-p-dimethylanilino-1-phenylethylene-2-yl)phthalide,
3-(p-dimethylanilino)-3-(1-p-dimethylanilino-1-p-chlorophenylethylene-2-yl)-6-dimethylaminophthalide,
3-(4xe2x80x2-dimethylamino-2xe2x80x2-methoxy)-3-(1xe2x80x3-p-dimethylanilino-1xe2x80x3-p-chlorophenyl-1xe2x80x3,3xe2x80x3-butadiene-4xe2x80x3-yl)benzophthalide,
3-(4xe2x80x2-dimethylamino-2xe2x80x2-benzyloxy)-3-(1xe2x80x3-p-dimethylanilino-1xe2x80x3-phenyl-1xe2x80x3,3xe2x80x3-butadiene-4xe2x80x3-yl)benzophthalide,
3,6-bis(dimethylamino)fluorenespiro(9,3xe2x80x2)-6xe2x80x2-dimethylaminophthalide,
3-dimethylamino-6-dimethylamino-fluorene-9-spiro-3xe2x80x2-(6xe2x80x2-dimethylamino)phthalide,
3,3-bis-[2-(p-dimethylanilino)-2-(p-methoxyphenyl)ethenyl]-4,5,6,7-tetrachlorophthalide,
3-bis[1,1-bis(4-pyrrolidinophenyl)ethylene-2-yl]-5,6-dichloro-4,7-dibromophthalide,
bis(p-dimethylaminostyryl)-1-naphthalenesulfonylmethane, and
bis(p-dimethylaminostyryl)-1-p-tolylsulfonylmethane.
In the thermosensitive recording material according to the present invention, the color developer comprises at least one compound selected from the group consisting of the previously mentioned compounds of formulas (I) to (IV). Furthermore, when necessary, other electron-acceptors, for instance, phenolic compounds, thiophenolic compounds, thiourea derivatives, organic acids and metallic salts thereof can be employed in combination with the compounds (I) to (IV). Specific examples of such electron-acceptors are as follows:
4,4xe2x80x2-isopropylidenebisphenol,
4,4xe2x80x2-isopropylidenebis(o-cresol),
4,4xe2x80x2-sec-butylidenebisphenol,
4,4xe2x80x2-isopropylidenebis(o-tert-butylphenol),
4,4xe2x80x2-cyclohexylidenebisphenol,
4,4xe2x80x2-isopropylidenebis(2-chlorophenol),
2,2xe2x80x2-methylenebis(4-methyl-6-tert-butylphenol),
2,2xe2x80x2-methylenebis(4-ethyl-6-tert-butylphenol),
4,4xe2x80x2-sec-butylidenebis(6-tert-butyl-2-methylphenol),
1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane,
4,4xe2x80x2-thiobis(6-tert-butyl-2-methylphenol),
2,4xe2x80x2-diphenolsulfone,
2,2xe2x80x2-diallyl-4,4xe2x80x2-dihydroxydiphenylsulfone,
3,4xe2x80x2-dihydroxy-4xe2x80x2-methyldiphenylsulfone,
4-isopropoxy-4xe2x80x2-hydroxydiphenylsulfone,
4-benzyloxy-4xe2x80x2-hydroxydiphenylsulfone,
4,4xe2x80x2-diphenolsulfoxide,
isopropyl p-hydroxybenzoate,
benzyl p-hydroxybenzoate,
benzyl protocatechuate,
stearyl gallate,
lauryl gallate,
octyl gallate,
1,7-bis(4-hydroxyphenylthio)-3,5-dioxaheptane,
1,5-bis(4-hydroxyphenylthio)-3-oxapentane,
1,3-bis(4-hydroxyphenylthio)-propane,
monocalcium salt of monobenzyl phthalate,
N,Nxe2x80x2-diphenylthiourea,
N,Nxe2x80x2-di(m-chlorphenyl)thiourea,
salicylanilide,
antipyrine complex of zinc thiocyanate,
zinc salt of 1-acetyloxy-2-naphthoic acid,
zinc salt of 2-acetyloxy-3-naphthoic acid,
zinc salt of 2-acetyloxy-1-naphthoic acid,
bis(4-hydroxyphenyl)methyl acetate,
bis(4-hydroxyphenyl)benzyl acetate,
4-[xcex2-(p-methoxyphenoxy)ethoxy]salicylic acid,
1,3-bis(4-hydroxyphenyl)benzene,
1,4-bis(4-hydroxyphenyl)benzene,
4,4xe2x80x2-diphenolsulfone,
3,3xe2x80x2-diallyl-4,4xe2x80x2-diphenolsulfone,
xcex1,xcex1-bis(4-hydroxyphenyl)-xcex1-methyltoluene,
tetrabromobisphenol A,
tetrabromobisphenol S,
4,4xe2x80x2-thiobis(2-methylphenol),
4,4xe2x80x2-thiobis(2-chlorophenol),
zinc p-nitrobenzoate,
1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)isocyanuric acid,
2,2-bis(3,4xe2x80x2-dihydroxyphenyl)propane, and
bis(4-hydroxy-3-methylphenyl)sulfide.
To obtain a thermosensitive recording material according to the present invention, a variety of conventional binder agents can be employed in the thermosensitive recording layer for binding the above-mentioned leuco dyes, color developers, and auxiliary components to be described later to the support of the thermosensitive recording material. Alternatively, when the intermediate layer is provided between the support and the thermosensitive recording layer, the plastic minute void particles may be bound and supported on the support. As the binder agent used to prepare the thermosensitive recording layer and the intermediate layer, any conventional binder agents used in the conventional thermosensitive recording materials can appropriately be employed.
Examples of the binder agent are water-soluble polymers such as polyvinyl alcohol, starch and starch derivatives, cellulose derivatives such as methoxy cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose and ethyl cellulose, sodium polyacrylate, polyvinyl pyrrolidone, acrylamidexe2x80x94acrylic ester copolymer, acrylamidexe2x80x94acrylic esterxe2x80x94methacrylic acid terpolymer, alkali salts of styrenexe2x80x94maleic anhydride copolymer, alkali salts of isobutylenexe2x80x94maleic anhydride copolymer, polyacrylamide, sodium alginate, gelatin, and casein; emulsions each as polyvinyl acetate, polyurethane, polyacrylic ester, polymethacrylic ester, vinyl chloridexe2x80x94vinyl acetate copolymer, and ethylenexe2x80x94vinyl acetate copolymer; and latexes such as styrenexe2x80x94butadiene copolymer and styrenexe2x80x94butadienexe2x80x94acrylic copolymer.
In the present invention, the thermosensitive recording layer may comprise a thermofusible material as the thermosensitivity-improving agent.
The specific examples of the above-mentioned thermofusible material are as follows: fatty acids such as stearic acid, and behenic acid; fatty amides each as stearic acid amide, and palmitic acid amide; fatty acid metallic salts such as zinc stearate, aluminum stearate, calcium stearate, zinc palmitate, and zinc behenate; and p-bensylbiphenyl, terphenyl, triphenylmethane, benzyl p-benzyloxybenzoate, xcex2-benzyloxy naphthalene, phenyl xcex2-naphthoate, phenyl 1-hydroxy-2-naphthoate, methyl 1-hydroxy-2-naphthoate diphenyl carbonate, guaiacol carbonate, dibenzyl terephthalate, dimethyl terephthalate, 1,4-dimethoxynaphthalene, 1,4-ethoxynaphthalene, 1,4-dibenzyloxynaphthalene, 1,2-bis(phenoxy)ethane, 1,2-bis(3-methylphenoxy)ethane, 1,2-bis(4-methylphenoxy)ethane, 1,4-bis(phenoxy)butane, 1,4-bis(phenoxy)-2-butene, 1,2-bis(4-methoxyphenylthio)-ethane, dibenzoylmethane, 1,4-bis-(phenylthio)butane, 1,4-bis(phenylthio)-2-butene, 1,2-bis(4-methoxyphenylthio)ethane, 1,3-bis(2-vinyloxyethoxy)benzene, 1,4-bis(2-vinyloxyethoxy)benzene, p-(2-vinyloxyethoxy)biphenyl, p-aryloxybiphenyl, p-propargyloxybiphenyl, dibenzoyloxymethane, 1,3-dibensoyloxypropane, dibenzyl disulfide, 1,1-diphenylethanol, 1,1-diphenylpropanol, p-(benzyloxy)benzylalcohol, 1,3-diphenoxy-2-propanol, N-octadecylcarbamoyl-p-methoxycarbonylbenzene; N-octadecylcarbamoylbenzene, dibenzyl oxalate, bis(4-methylbenzyl)oxalate, bis(4-chlorobenzyl)oxalate, 1,5-bis(p-methoxyphenyloxy)-3-oxapentane, and 1,2-bis(4-methoxyphenoxy)propane.
When necessary, the thermosensitive recording layer for use in the present invention may further comprise auxiliary additive components such as a filler, a surface active agent, a lubricant and an agent for preventing color formation by pressure application, which are used in the conventional thermosensitive recording materials.
Examples of the filler for use in the present invention are finely divided particles of inorganic fillers such as calcium carbonate, silica, sine oxide, titanium oxide, aluminum hydroxide, zinc hydroxide, barium sulfate, clay, kaolin, talc, surface-treated calcium and surface-treated silica; and finely-divided particles of organic fillers such as urea-formaldehyde resin, styrene-methacrylic acid copolymer, polystyrene resin and vinylidene chloride resin.
Examples of the lubricant for use in the present invention include higher fatty acids and amides, esters and metallic salts thereof; and a variety of waxes such as an animal wax, a vegetable was, a mineral wax, and a petroleum wax.
The thermosensitive recording material of the present invention may further comprise an additional layer comprising a pigment, a binder agent and a thermofusible material when necessary, which is provided between the previously mentioned intermediate layer and the thermosensitive recording layer.
Furthermore, the thermosensitive recording material may further comprise a protective layer which is provided on the thermosensitive recording layer in order to improve the preservation stability of the recorded images and the writing quality of the recording material. The protective layer comprises the previously mentioned pigment, binder agent, and thermofusible material.
The color developer for use in the present invention comprises at least one compound selected from the group consisting of the previously mentioned compounds of formula (I) to (IV) as shown in Tables 1 to 4. Among these compounds, the compound No. I-3 and the compound No. I-29 shown in Table 1 are novel compounds and superior is the color development performance In addition, the preservation stability, particularly, the resistances to fete and vile, and plasticizers of both an image portion end a background portion of the recording material is improved by using these compounds No. I-3 and No. I-29 as the color developers in the thermosensitive recording material.
The method of synthesizing the compound of formula (I) will now be explained with reference to the synthesis of the compound No. I-3.
The compound No. I-3 can be prepared by the conventional reaction, for instance, in accordance with the following reaction scheme (1), (2) or (3): 
In the above-mentioned reaction schemes (1) to (3), X is a halogen atom. The previously mentioned compounds of formulas (II) and (IV) serving as the color developers in the present invention can also be synthesized in accordance with the reaction schemes similar to those as mentioned above. To synthesize the compound of formula (III), the reaction schemes (1) and (2) are applicable because the synthesis reaction including the process of hydrolysis is not preferable.
More specifically, the compound No. I-29 can be synthesized in the following manner: dimethyl 4-hydroxyphthalate is allowed to react with 4-halogeno-1-phenoxybutane in the presence of an alkali. The reaction product thus obtained is subjected to hydrolysis in the presence of an alkali, so that the compound No. I-29 can be prepared. In the above-mentioned reaction, potassium hydroxide and sodium hydroxide are preferably employed as the alkali compounds. As the halogen substituent of the above-mentioned phenoxybutane derivative, bromine and chlorine are preferably employed. Any solvents is which dimethyl 4-hydroxyphthalate and 4-halogeno-1-phenoxybutane can be dissolved and a small amount of an alkaline aqueous solution can be uniformly dissolved are preferably as the solvent for use in the above reaction. For instance, ethanol, methanol and dimethyl sulfoxide are preferably employed as the solvent. In this case, the use of a high boiling point solvent can curtail the reaction time. A solvent in which the materials cannot completely be dissolved may also be employed in such a condition that the materials are dispersed therein, or it may be used in combination with a good solvent. When necessary, a catalyst capable of transferring between an organic phase and a water phase, for example, quaternary ammonium salts such as triethylbenzylammonium chloride, tetramethylammonium chloride and tetramethylammonium bromide can be employed.
A substituted methyl phthalate obtained by the reaction is subjected to hydrolysis in the presence of as alkali, so that a desired compound can be obtained. The hydrolysis reaction may be carried out using a solvent in which both the substituted methyl phthalate and water are dissolved, or without any solvent. In the case where no solvent is employed, it is desirable to carry out the hydrolysis reaction in a large quantity of water containing sodium hydroxide or potassium hydroxide in an amount of about 5 to 10 wt. % at a temperature as high as about 100xc2x0 C. In this case, it is preferable to use 3 moles or more of the alkali compound to 1 mole of the substituted methyl phthalate in the hydrolysis reaction.
In the case where the hydrolysis reaction proceeds in a large quantity of alkaline water without any solvent, the alkaline water becomes colorless and transparent when a half ester is obtained. It is preferable to heat the alkaline water alter the alkaline water becomes transparent in order to complete the hydrolysis reaction. After the completion of the hydrolysis reaction, an acid such as hydrochloric acid or sulfuric acid which is appropriately diluted is added dropwise to the alkaline solution, so that a desired product separates out. The reaction mixture is dispersed for several hours with the pH of the reaction mixture maintained at about 3 while heating to about 60 to 70xc2x0 C.
Then, the substitution is computed and the desired product separates out.
Then, the obtained product is purified when necessary. It is preferable to purify the product by the recrystallization from a solvent such as water, alcohol, toluene, ethyl acetate, hexane or a mixed solvent thereof.